Membranes can be incorporated into optical devices, such as telescopes, to produce optical systems with significant mass reductions compared to optics using conventional mirror or lens technology. For example, a membrane manufactured with a diffractive focusing capability can be employed as a primary diffractive optical element in a space telescope. Membranes in space will tend to reach very cold equilibrium temperatures due to natural thermal radiation to cold space. These cold temperatures can result in the build-up of significant stress in the membrane, with the result that the membrane may tear or become permanently distorted. Even in the absence of the potential for permanent physical changes, a membrane with focusing capability that changes dimension as a function of temperature will exhibit significant differences in optical performance as a function of temperature. Correcting these changes increases the optical, mechanical, and dynamic control complexity of the optical system. Accordingly, some form of heating and temperature control is required to maintain the membrane's physical and dimensional integrity for space-based large optical systems. In addition, any heating approach must not significantly impact the optical performance, including optical throughput, of the optical system. However, controlling the temperature of a large membrane is difficult, particularly where the membrane substrate is extremely thin (e.g. about 20 microns) and has poor thermal conductance. Moreover, any approach to heating a membrane in space should heat the membrane evenly to avoid dimensional distortion. However, previous techniques available for heating circular or irregular membranes have not allowed for even heating of such membranes.
For non-optical membranes, heating and temperature control can be accomplished by incorporating heating grids into or on the surface of the membrane. However, for optical membranes, and in particular for transmissive optical membranes where structure within the optical aperture compromises the membrane's optical performance, the addition of heating grids is not practical. Transparent thin film heaters, such as Indium Tin Oxide (ITO) films, have been used for very small components where the thin film can be applied to a rectangular window. However, ITO-based heating has not been applied to meter-scale membrane structures or to meter-scale circular optical membranes. In particular, conventional ITO-based heating systems and techniques will not be effective to evenly heat a relatively large, irregularly shaped membrane, such as, but not limited to, a section of an optimal element having an overall diameter of 1 meter or larger. Radiative proximity heaters can work for components that can be surrounded, or for components with good thermal conductance. However, proximity heaters will not work for optical membranes if those heaters or components thereof block the membrane aperture from passing light properly. In addition, proximity heaters will be very inefficient if they are located outside of the membrane aperture.